Frequently, a fluid transfer device, such as a tubular conduit, is subjected to mechanical stresses such as thermal expansion due to external forces. In a tubular conduit structure, pulling along the tube axis at the ends of the tubular conduit that results from thermal expansion causes tensile stresses in the conduit structure, while pushing causes compressive stresses. Often an excessive amount of such stresses leads to a structural failure of the conduit. Further, in some applications, heat transfer between fluid flowing through the conduit and the surrounding environment can be problematic. For example, insulating a steam pipe to reduce heat loss of steam as it flows through the conduit has been a challenge.
A fluid transfer conduit is also utilized in fuel nozzles for conventional gas turbine engine systems. Often fuel systems for regulating a flow of fuel to a combustion chamber of gas turbine engines include one or more fuel nozzles arranged in the combustion chamber, a fuel pump for pressurizing fuel from a fuel supply, a fuel metering unit for controlling the flow of fuel to the fuel nozzles, and one or more fuel manifolds fluidically connecting the fuel metering unit to the fuel nozzles. During engine start-up, fuel is pumped from the fuel supply to the fuel metering unit by the fuel pump and, once a sufficient start-up pressure is attained, a pressurizing valve of the fuel metering unit opens and fuel is supplied to the fuel nozzles via the fuel manifold. In the fuel nozzles, fuel is transferred through a fuel transfer conduit and injected into the combustion chamber.
Due to severe operating conditions within gas turbine engines, the engine's fuel nozzles are required to satisfy numerous design challenges. One such challenge posed by severe external pressures and temperatures of hot compressor discharge air surrounding the exterior of the fuel nozzle is accommodation of large thermally induced deformations within the body of the fuel nozzle. Unfortunately, a conventional fuel transfer conduit of the fuel nozzle comprising a single tube can experience a high thermal stress in such severe operating conditions, which results in structural failures.
Another challenge is thermally shielding fuel from the severe external temperatures. It is desirable to deliver fuel at a much lower temperature than the surrounding hot compressor air during turbine engine operations. If too much heat is transferred to fuel, fuel can begin to coke, thereby ruining or reducing the quality and delivery of fuel. Thus, conventional fuel nozzle designs utilize various insulating schemes to reduce the amount of heat that can be transferred from the high-temperature compressor air to fuel passing through the fuel nozzle. For example, a gap between the fuel transfer conduit and the fuel nozzle support is filled with an air-fuel-coke mixture, which has a lower thermal conductivity than a constituent metal of the fuel nozzle support, thereby shielding fuel from the environment external to the fuel nozzle support. However, the evolution of the gas turbine engines has been such that temperature and pressure of the compressor discharge air have substantially risen, and thus, the thermal shielding of fuel has become even more of a design challenge. For many applications, it has been found that the conventional single insulating fuel gap filled with the air-fuel-coke mixture does not provide enough thermal protection to fuel that the engine manufacturers require.
In view of these challenges, there is a need in the art for an improved fluid transfer device. The present invention pertains to such improvements to the state of the art of a fluid transfer conduit which can better accommodate external stresses and insulate fluid from external environments.